An optical fiber used as a signal transmission line in an optical transmission system has wave dispersion characteristics. The term “chromatic dispersion” is a phenomenon in which the transmission rate of light varies depending on the wavelength thereof and acts as a cause of distorting an optical signal. The longer the transmission distance, the more the chromatic dispersion increases. As a result, the distortion of a waveform increases. The optical receiver, which receives an optical signal, has a dispersion tolerance (dispersion proof characteristic). The dispersion tolerance of the optical receiver is specified by the residual amount of chromatic dispersion for satisfying a regular transmission penalty.
The farther the residual amount of chromatic dispersion is from the optimal value, the more limited the transmission distance of the optical transmission system is by the chromatic dispersion. The dispersion tolerance becomes smaller in inverse proportion to the square of the ratio of transmission rate (e.g., bit rate). On the reception side of an optical signal, a chromatic dispersion compensator (hereinafter, simply referred to as a “DC”) may be provided having the inverse characteristic of the chromatic dispersion of a transmission line to compensate the chromatic dispersion caused in the transmission line.
In addition, the optical transmission system based on wavelength division multiplexing (WDM) realizes long distance transmission by performing collective compensation (waveform-distortion correction) on optical signals of the respective wavelengths (optical signals of the respective channels) included in a WDM signal using the DC. The examples of the DC include a fixed DC in which the compensated amount of chromatic dispersion is a fixed value and a tunable optical DC (hereinafter, simply referred to as a TODC) in which the compensation amount of chromatic dispersion is variable and externally controlled.
Conventional optical transmission systems predominantly use a fixed DC in which the amount of chromatic dispersion is fixed. The dispersion property of the fixed DC is designed to have the reverse characteristic of the chromatic dispersion of the transmission line. In this case, however, the complete reverse characteristic is not observed. The use of the fixed DC causes an error in dispersion compensation due to any of various factors, so that the long distance transmission may lead to insufficient compensation or over compensation.
In a high-bit rate signal transmission with a narrow dispersion tolerance of 40 Gbps or more, chromatic dispersion serves as a significant limiting factor. Thus, a conventional WDM transmission system at a high rate of 40 or more Gbps using the TODC in which a different compensation amount is set to each channel (wavelength) is described in for example, Japanese Laid-open Patent Publication No. 2007-329558.
A TODC may be replaced with the fixed DC or may be used in combination with the fixed DC. TODC suppresses the compensation error between channels (wavelengths). In addition, when a change in amount of chromatic dispersion generated by an optical signal in the transmission line occurs, the degradation of a transmission characteristic may be prevented by adjusting the compensation amount of chromatic dispersion to an optimal level. The change in amount of chromatic dispersion may be recognized by monitoring the number of errors or bit error rate (BER) of the optical signal.